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structure due to variations in polymer blending or adhe-

sion of various layers in a laminate can often be extremely

complex, time-consuming, or impractical for commerciali-

zation. The development of a successful perforation pro-

tocol for these ﬁlms is usually driven by knowledge of the

interaction of laser energy with ﬁlm structural compo-

nents, and then whether or not the perforated product can

be marketed as a packaging solution at a cost that would

suit the utilizing industry.

BIBLIOGRAPHY

1. S. Ben-Yehoshua, S. Fishman, S. Jayanty, N. Mir, and R. M.

Beaudry, ‘‘Modiﬁed Atmosphere Packaging, Controlled Atmo-

sphere Packaging, and Seal Packaging’’ in S. Ben-Yeshoshua,

ed., Post Harvest Physiology, Shefﬁeld Academic Press, Shef-

ﬁeld, UK, 2005, pp. 61–112.

2. Y. Mizutani, Ind. Eng. Chem. Res. 32, 221–227 (1989).

3. G. Peiser and T. V. Suslow, Perish. Handl. Q. 95, 17–19 (1998).

FILM, PLASTIC

ALLEN G. KIRK

E. I. Du Pont de Nemours, Inc.,

Wilmington, Delaware

Films are continuous membranes that can separate one

area from another. These membranes can vary in thick-

ness, ranging from less than that for rigid containers, to

sheeting, to ﬁlm, and even to thin coatings. Usually ﬁlms

are considered self-supporting and less than B10 mils

(250 mm) in thickness. No deﬁnition is given for a mini-

mum thickness, which can vary depending on the material

of construction of the ﬁlm; however, thicknesses of o1 mil

(25 mm) are common.

An important feature of most ﬁlms discussed in this

article is heat sealability, which refers to the thermoplastic

property of the ﬁlm, or coating on the ﬁlm, which allows it

to be fashioned into pouches, bags, and overwrapped

packages by virtue of its ability to make a hermetic seal

to itself. Heat sealing is accomplished by heating up the

ﬁlm areas then applying the hot areas to each other under

pressure. Sometimes these operations occur simulta-

neously. During heat sealing, the polymer molecules be-

come entangled; the better the intermingling, the stronger

the seal. The time allowed for heat sealing in a typical

high-speed food-packaging machine is less than 1 s, during

which time heat-seal strengths of very high values can be

achieved, i.e., several pounds per inch of seal width.

FILM USES

Because of their ability to keep two areas (or volumes)

separated from each other, ﬁlms have come to have a

multitude of commercial uses over the years, particularly

in the area of food packaging. Films do provide protection

to the foodstuffs being displayed, and in doing so they can

provide a multitude of other functional attributes. They

can function as barriers for gases, vaporous ﬂavor compo-

nents, or moisture from escaping from the food, thus

preserving its freshness. They can also protect the food-

stuff from attack by undesirable outside agents such as air

(oxygen), moisture, or sunlight, thus ensuring that the

food does not become rancid or soggy. The ﬁlm material

can serve as a partial gas barrier, thus allowing some

gases to escape but not others; the choice of packaging ﬁlm

Stand 10.05.2007 CO

Systems – The Perforation People

Focus on Fine Solutions

-1 -

PERFOLAS

Details - Perforation Area

Individual adjustment

of focus

16 beams with

independent shutters

Individual adjustment

of head position

High-power exhaust system with filter

Laser safety class 1 protection housing

Individual selection of the number of active

perforation heads

Figure 1. Laser system for manufacture of perforated ﬁlm.

488 FILM, PLASTIC

to protect fresh vegetables and fruit can be very selective,

especially if the vegetables and fruit are already cut or

sliced for customer convenience. For fresh produce, the

ﬁlm must allow CO

to easily leave the package while

allowing only low levels of air (oxygen) to enter the

package for longer shelf life.

Films also provide a billboard for information about the

contained product, directions on how to open the package,

recommended dose levels, warnings about toxic contents,

nutritional information, and universal product code (UPC)

symbols for pricing and inventory control. This informa-

tion can be displayed in a multicolor and aesthetically

pleasing way on the packaging ﬁlm so that it ‘‘sells’’ itself

and acts as an impulse-buying aid. In addition, the

transparency and high clarity of the ﬁlm can provide a

window in the package to allow the consumer to judge

desirability of the product before buying or to judge

remaining quantity in the package after purchase and as

it sits in the consumer’s home.

Film applications can be large, as in the case for some

construction uses. Films or sheeting materials are used to

wrap houses and buildings under construction, thus ser-

ving as moisture and wind barriers in the ﬁnished con-

struction. Film products can be used in highway

construction, as leakage barriers in municipal water

reservoirs and as protective liners for municipal solid-

waste landﬁll sites. Film applications can be small, such

as the push-through blister pack for individually pack-

aged medical pills. Packaging ﬁlm uses can be categorized

as follows.

Overwrap. This type of packaging usually involves

unwinding a roll of ﬂat ﬁlm into the packaging machine,

where it is folded around the object, the ﬁlm sealed to

itself, and the ﬁnished package removed by cutting the

ﬁlm. Typical products packaged by this method include

small packs of snack crackers and cookies. The machine

is commonly called a ‘‘horizontal make-and-ﬁll machine.’’

Products such a potato chips, corn chips, and bags of candy

are packaged in vertical form/ﬁll/seal machines in which a

roll of ﬂat ﬁlm feeds into the machine over a forming collar

where the ﬁlm is formed into a tube. This tube is sealed

along the back with a vertical heat seal, and then the

bottom cross seal is made. The open-top tube is ﬁlled with

the product and indexed down, and the top cross seal is

made simultaneously as the bottom seal is formed for the

next pouch to be ﬁlled and as the ﬁlled pouch is removed

from the packaging machine. Coated cellophane, coated

oriented polypropylene (OPP) ﬁlm, and laminated ﬁlms

typically are used for this packaging application.

Skin Packaging. As the term implies, this type of packa-

ging involves forming a skin of ﬁlm over the object being

packaged and sealing the skin ﬁlm tightly to a heat-seal-

coated display card. This is accomplished by placing the

object to be packaged, usually a hardware item, onto the

coated display card, then draping a hot sheet of ﬁlm over

the entire assembly, and applying vacuum to the backside

of the permeable display card. The vacuum pulls the

cooling hot ﬁlm down over the object and applies pressure

to assist in making the seal to the display card. Typically

ionomer, low-density polyethylene (LDPE), or polyvinyl

chloride (PVC) resin ﬁlms are used for this packaging

applications, and the board coating is LDPE or ethylene-

vinyl acetate (EVA) (See the Skin packaging article).

Blister Packaging. Preformed blisters are ﬁlled with the

object, the coated paperboard is placed over the ﬁlled

blister, and the assembly is heat sealed. Small hardware

items and small notions are commonly packaged in this

manner.

Shrink Packaging. This type of packaging refers to the

use of a ﬁlm manufactured in such a way that when it is

heated it will contract in both directions, reducing its

surface area. When this type of ﬁlm is wrapped around

an object and sealed around its edges, then sent through a

shrink tunnel where large volumes of heated air wash

over it, the ﬁlm will react to the heat and contract down to

the object, thus making an attractive skin-tight package.

A vent hole must be cut into the ﬁlm to allow the trapped

air to escape from the package as is undergoes ﬁlm

shrinkage in the hot-air tunnel. Shrink-ﬁlm resins are

polyethylenes, polypropylenes, PVC, polyesters, and some

coextruded structures. Articles packaged in this manner

are numerous and can include frozen poultry, pizzas,

stationery, toys, cassettes, and compact disks.

Stretch Packaging. This packaging technique is useful

for bundling objects together for convenience in shipping

or handling. Resins used in this application include linear

LDPE (LLDPE), LDPE, EVA, PVC, and some copolymers

of polypropylene. Typically, when stretch wrap is used for

pallet bundling, the tacky ﬁlm is wound around the pallet

many times to ensure tightness, the ﬁlm cut, and the ﬁlm

end stuck down on the wrapped pallet. There is no need to

heat seal or use mechanical fasteners with this packaging

technique.

PLASTIC-FILM RESINS

It would not be possible to cover all resins that have been

fabricated into ﬁlm in this article. What are presented

here are typical resins used over the years as packaging

ﬁlms.

Food packaging has successfully employed ﬁlms for

many years, starting in the 1920s in the United States

with the introduction of cellophane ﬁlm, which was used

initially to package loaves of bread that had been pre-

viously overwrapped in paper. Because it was transpar-

ent, cellophane allowed the bread to be viewed by the

customer before purchase. Uncoated cellophane is com-

posed

of regenerated cellulose

, a softening agent, and

water as a plasticizer. Although it is not moistureproof,

it is impermeable to gases when the ﬁlm is kept dry.

Additional slip, antistatic, and antifog properties can be

provided by special formulations applied as coatings to the

base sheet. Cellophane is not a thermoplastic and does not

have a melting point, heat sealability is possible only by

applying a heat-seal coating such as nitrocellusose (NC) or

polyvinylidene chloride (PVDC) to the base sheet.

FILM, PLASTIC 489

By the early 1950s, LDPE resin was developed and

began serving as the ﬁrst major human-made (synthetic)

thermoplastic packaging resin. Because this resin has a

melting point and is thermoplastic, the packaging indus-

try developed thermal extrusion equipment to extrude

thin ﬁlms of this new resin for packaging. Polypropylene

(PP), which is another popular packaging resin, was ﬁrst

produced during the 1950s. However, it did not develop

commercially until the 1970s, when new high-efﬁciency

catalysts were discovered for its production. PP competed

effectively for many of the cellophane packaging applica-

tions. Although cellophane had attractive properties of

clarity, stiffness, gas barrier, and heat resistance, it also

had disadvantages of limited shelf life because of the loss

of volatile plasticizer. This loss of plasticizer caused the

cellophane to embrittle to the point where the packaged

product was no longer protected. PP has the inherent

attributes of clarity, toughness, thermoplasticity, and heat

sealability after being coated by PVDC. PP also has the

advantage of having a density lower than that of cello-

phane; thus, more square inches of packaging ﬁlm are

possible from a pound of PP than from a pound of

cellophane ﬁlm—a deﬁnite economic advantage. As PP is

a thermoplastic resin, it did not have the thermal resis-

tance to provide fail-safe high-temperature sealing protec-

tion against substrate burn through on the packaging

machine. Additionally, the lower modulus PP could not

be fed easily through the push-feed packaging equipment

developed years earlier for the stiffer cellophane. Feeding

problems for PP were exacerbated by its greater tendency

to generate and hold static charge, which caused clinging

and feeding jams in the packaging machine. These serious

packaging problems were eventually eliminated by ma-

chinery redesign and by improvements in PP base-sheet

and coating technology. Plastic-ﬁlm resins can be categor-

ized by the chemical process used to form the molecules in

the polymer resin backbone.

Addition Polymerization (Homopolymers). Polymers in

this category include LDPE, HDPE, PP, polybutylene

(PB), and polystyrene. During the polymerization or

building of the polymer molecules, the individual mono-

mer units (ethylene, propylene, and butylene, respec-

tively) are chemically connected together in the pressure

reactor in the presence of high temperature and an

appropriate catalyst. By control of the residence time in

the reactor, the number of these monomer connections can

be varied. Long reactor time, longer polymer chain length,

and higher molecular weight result. By controlling mole-

cular weight, the manufacturer can control polymer resin

properties such as viscosity, which, in turn, can control

ﬁnal ﬁlm properties such as toughness. These resins,

called aliphatic resins, are prepared from monomers that

consist of only carbon and hydrogen atoms. The character-

istic chemical structure in the polymer backbone is a

carbon-to-carbon bond (—CH

—CH

—) that links the

individual monomer units together. Aliphatic ethylene

and propylene homopolymer resins are nonpolar, have

good clarity, and have relatively low melting points that

range from B1051C for LDPE to 1251C for high-density

polyethylene (HDPE), and 1651C for PP. Polystyrene does

not have a melting point but does exhibit a Vicat softening

point at T r 1061C. Because their monomers are derived

relatively easily from petroleum feedstocks and the corre-

sponding resins are used in large-volume packaging ap-

plications, resin cost/pound is not high, particularly for

LDPE, HDPE, and PP. LDPE resins can be found, for

example, as premade ﬂexible bags and pouches and other

large-volume packaging applications. HDPE, which is

stiffer and tougher, is used as grocery bags and multiwall

bags and bag liners. PP, as described above, has replaced

cellophane ﬁlm in many food-packaging applications.

Some of these applications include overwrap for cigarette

packs, snack-food packs, and pouches and bags for potato

and corn chips. PB has found some applications as a meat-

packaging ﬁlm and as an additive for hot-melt adhesives.

Addition Polymerization (Copolymers). Polymers pre-

pared from combinations of two or more monomers can

give rise to plastic resins with a wide range of properties

not possible from homopolymers. Resins of ethylene/propy-

lene (E/P), ethylene/butylene (E/B), ethylene/hexene

(E/H), and ethylene/octene (E/O) are possible combinations.

The last three mentioned resins are gaining popularity as

new aliphatic polymers with a much broader range of

thermal and mechanical properties than previously possi-

ble. These resins, which are designated metallocene poly-

ethylenes (mPEs) are prepared using the relatively new

constrained geometry catalysts that allow the tacticity and

branching of the polymer molecule to be better controlled,

which yields polymers with narrower molecular distribu-

tions and lower densities. mPE resins are aliphatic copoly-

mers somewhat endowed with the properties of the higher

polarity copolymers described below.

Addition copolymers can also be prepared with ethy-

lene and a polar second monomer. These polar monomers

can include acrylic acid (AA), methacrylic acid (MAA),

ethyl acrylate (EA), and vinyl acetate (VA). Because these

monomers contain oxygen atoms in addition to carbon and

hydrogen and because oxygen is a heavier atom and rich

in electrons, the polymerization of these polar monomers

results in a copolymer resin with higher polarity than

carbon- or hydrogen-containing aliphatic resins. Resins of

E/AA, E/MAA, E/EA, and E/VA are characterized not by

an unusual chemical linkage between the repeated mono-

mer units but rather by the polar nature of the copolymer

in the polymer chain with the ethylene monomer. The

E/AA and E/MAA resins, which are also called acid

copolymer resins, have good clarity, low haze, and lower

melt and sealing temperatures, and they can adhere

strongly to polar substrates such as paper, foil, and some

highly polar ﬁlm resins. Thus, they have found use more

in specialty packaging applications and in association

with multilayer structures for meat, cheese, snack foods,

and medical items. E/VA resins typically contain between

5% and 40% VA comonomer, with VA contents above about

25% used largely in hot-melt adhesive applications.

Packaging-ﬁlm applications employ resins with lower VA

contents ranging from 5% to about 18%. These composi-

tions are not too sticky, and ﬁlms of these compositions can

be handled on typical packaging machines; however, often

slip and antiblock additives must be incorporated into the

490 FILM, PLASTIC

resin to ensure the proper slip level. Because of the low

crystallinity, toughness properties at low temperatures,

and low melting points, E/VA resins are used as a poultry

and meat wrap, bag-in-box for liquid packaging, stretch

ﬁlm, and as ice bags.

Because E/AA and E/MAA resins contain free-acid

groups, they can be partially neutralized to form a class

of resins called ionomers. The ionomers serve some of the

same market applications and have excellent hot-tack

sealability, the ability to seal through contamination,

and grease/oil resistance. These polar copolymer resins

ﬁnd use in packaging meat, cheese, breakfast cereals, and

many other wet and dry products. They also are used as

skin-packaging ﬁlms for hardware products.

Condensation Polymerization. Some chemically reactive

polar monomers can react with each other or with a second

monomer to form a polymer chain and a small volatile

second molecule. Many times that second molecule is

water; thus, water seems to ‘‘condense’’ from the reaction.

Removal of this second molecule leaves the relatively

clean polymer behind. Resins made by this condensation

polymerization route include polyamides or nylon, and

polyester. Nylon resins are made from diamines and

dibasic acids and are characterized by the amide group

(—CONH—) in the polymer backbone. Nylon resins are

identiﬁed by the number of carbon atoms in the mono-

mers. Thus, homopolymer nylon 6 can be prepared from a

monomer with six carbons in a straight line with reactive

end groups on each end. Also nylon 6/6, 6/10, and 6/12 can

be prepared from two monomers with the differing num-

ber of carbon atoms. Polyamides are regarded as semi-

crystalline resins with high melting points (175–2751C).

As packaging ﬁlms, they offer toughness, chemical resis-

tance, resistance to oils and greases, and moderate gas

barrier. Food products packaged in polyamide ﬁlms in-

clude meats and cheeses.

Polyesters similarly are formed by reaction between an

aromatic diacid such as terephthalic acid or the dimethyl

ester of terephthalic acid and a polyalcohol such as

ethylene glycol. These resins are characterized by the

ester group (—CO—O—) linking the monomer units in

the polymer backbone. Thus, these resins contain a stiff

aromatic molecule within the polymer backbone as well as

a ﬂexible aliphatic portion. This stiff resin can be extruded

into oriented high-tensile-strength ﬁlm, metallized, and

heat-seal coated to yield a packaging ﬁlm in a thin gauge.

In combination with other sealant coatings or lamina-

tions, polyester ﬁlms are used as boil-in-bag pouches and

for processed-meat packaging.

Chlorinated Vinyl Addition Polymerization. Polyvinyl

chloride resin for ﬁlm applications is prepared by suspen-

sion polymerization and employs the vinyl chloride

(CH

QCHCl) monomer. A homopolymer resin is polymer-

ized from this monomer by a free-radical process. After

purifying and removing excess water, this rigid resin must

be blended with softeners or plasticizers to reduce its

glass-transition temperature and render it a usable resin.

For a more detailed discussion on how to modify melt

temperature and glass-transition temperature in poly-

mers, refer to Nielsen (1). Other additives such as lubri-

cants and thermal stabilizers are blended into the resin as

well. Packaging applications for PVC ﬁlm include fresh

meat, shrink ﬁlm, and blister packs. For more details on

how packaging ﬁlm choices are made for different food-

stuffs, refer to Jenkins (2).

MULTILAYER FILMS

Laminated ﬁlm structures refer to multilayered structures

composed of many monoﬁlm layers. By effecting adhesion

between the layers, more sophisticated ﬁlm properties can

be obtained in a single ﬁlm. Heat-sealable ﬁlms can provide

the outside layers and can cover interior layers of non-

thermoplastic materials such as foil, metallized ﬁlm, or

paper. Previously made ﬁlms that contain light-blocking

levels of TiO

can be printed before being laminated with a

clear glossy, heat-sealable ﬁlm, thus yielding an attractive

multilayered ﬁlm that can be heat sealed, providing protec-

tion of the product from ultraviolet (UV) degradation, and

exhibiting striking printed panels. Because each monoﬁlm

of the laminate must have been previously prepared in-

dividually as a self-supporting ﬁlm, the ﬁnal multilayered

laminates can end up thick and costly.

Coating of monoﬁlms also provides a higher level of

property sophistication to monoﬁlms. Frequently, solvent

coatings are applied to ﬁlms to provide heat sealability or

an enhanced gas and moisture barrier to the ﬁlm. Vacuum

metallization can be considered a form of coating. This

technique enhances light opacity and gas- and moisture-

barrier properties of the ﬁlm. Individual coating thick-

nesses are usually less than 1 mil (25 m m); thus, coating a

monoﬁlm does not substantially add to its thickness.

Coextrusion describes a process in which a multi-

layered ﬁlm is prepared by adhering several individual

ﬁlm layers within the body of the extrusion die. This

process can prepare as many as nine separate layers of

ﬁlm within the same ﬁnal ﬁlm. Each resin may have its

own extruder feeding a melt stream into the complex feed

block attached to the die. Sometimes, one extruder will

feed melt into two or more separate ﬂow streams by the

use of a stream divider within the feed block. This practice

is fairly common for the extruder supplying the tie layer

resin, which is the resin used to adhere two dissimilar

resin types in the ﬁnal multilayered ﬁlm. Many factors

must be considered when choosing the tie layer resin for a

coextruded structure (3). Obviously, the initial cost for

coextrusion equipment is high. However, this technique is

popular and can provide outstanding ﬁlm property combi-

nations at a reasonable cost.

BASIC FILM–FORMING PROCESSES

Thermoplastic resins such as ethylene and propylene

homopolymers and copolymers, and the condensation

polymers described above, by deﬁnition have a melting

temperature above which they are not rigid solids but

FILM, PLASTIC 491

rather viscoelastic materials. As such, they are soft plastic

melts that can be formed into shapes convenient for

packaging and other commercial uses. Several typical

thermal processes are available for effecting this conver-

sion from the small pellet supplied in the resin bag, box,

hopper truck, or hopper railroad car to the ﬁnal ﬁlm

structure. The following descriptions describe the pre-

paration of monoﬁlms; however, keep in mind that the

coextrusion process is also possible for all these extrusion

processes. Refer to Figure 4, which relates typical EVA

resin melt ﬂow requirements to ﬁlm manufacturing pro-

cess type.

Extrusion-Cast Film. In this process, the plastic resin

pellets are introduced into the feed hopper of the extruder,

where they are funneled down into the extruder barrel

and onto the rotating screw (see Figure 1). As the screw

rotates, it drives the pellets deeper along the hot barrel,

compressing and heating them and applying force or shear

so that the air spaces between the pellets are driven out

and the pellets are heated to the melt temperature or

softening temperature of the resin. The screw, which is

driven by a powerful electric motor, is typically composed

of three separate regions: (a) the compression zone, which

is closest to the feed hopper; (b) the transition zone; and

zone. Each zone occupies about one third of the screw

length. The compression zone heats up the pellets driving

out the air, the transition zone describes the length of the

screw where the transformation from solid pellet to

melted resin takes place, and the metering zone allows

the melt pool to become homogenized so that a uniformly

mixed melt stream is presented to the heated die. The die

serves the purpose of ﬁxing the dimensions of the ﬁnal

shape of the polymer. Thus, most dies have a circular cross

section to the melt inlet, which then gradually transforms

to the thin and wide cross section of the ﬁlm. As the ﬂat,

thin sheet of polymer melt exits the die, it is quenched to a

lower temperature below its melting point, thus solidify-

ing it into the ﬁnal and desired shape. Often the quench-

ing is accomplished with water-cooled metal drums or

rolls with a high-gloss chrome-plated and smooth ﬁnish

so that the quenched surface on the ﬁlm is ﬂat and

blemish-free. By controlling the takeoff speed of the

quench roll at a faster rate than what the melt exits the

die, the ﬁlm thickness can be reduced and controlled. This

differential in speed also impacts some ﬁnal ﬁlm proper-

ties, such as toughness, stiffness, and ﬁlm clarity or haze.

Basically, this stretching of the ﬁlm melt as it is cooling

causes the polymer molecules to become better aligned or

oriented in the long direction of the ﬁlm machine direction

(MD). This molecular alignment is responsible for en-

hance stiffness and toughness properties. The ﬁlm is

then accumulated as rollstock on the windup turret of

the cast-ﬁlm line. Film widths of r in 120 in. are possible

by this process. The length of the ﬁlm is deﬁned by how

much is wound on the roll, and usually as one ﬁlm roll is

being completed, the windup turret can be indexed around

so that a new roll can be started without interrupting the

continuous resin ﬂow through the extruder and die. The

rolls are then removed from the windup turret and slit to

the width desired by the customer. Property-enhancing

lubricants, stabilizers, or colorant additives can be added

to the resins pellets at the feed hopper. They will become

mixed with the melt stream by the extruder screw. These

additives can be added as powders to the feed hopper or

more likely will be added as a preblended concentrate in

pellet form.

Extrusion-Blown Film. The blown-ﬁlm process (Figure 2)

is somewhat similar to the cast-ﬁlm process in that a

plasticating screw extruder is employed to convert the

solid resin pellets into a uniform plastic melt. The blown-

ﬁlm process employs a circular or annular die to form the

ﬁlm, which is then stretched over and around a captured

bubble of air. The ﬁlm is usually air quenched, collapsed as

a ﬂat tube, and wound up on the windup stand. Some-

times, the bubble is slit in the vicinity of the nip rolls so

that two sheets of ﬁlm may be fed down to the windup

stand. This bubble process can be oriented so that the ﬁlm

extrudes upward, downward, or in the horizontal direc-

tion. The bubble may be long but typically is 20–30 ft long.

Bubble diameters may be 6 ft or more. The resin used in a

blown ﬁlm process must have a high melt strength to

sustain the relatively high hoop stress and MD tension.

This process requires that the melt emerge from the die

then quickly ﬂare out or inﬂate around the captured air

Figure 1. Cast-ﬁlm line.

Figure 2. Blown-ﬁlm line.

492 FILM, PLASTIC

bubble while it is being cooled. Thus, the polymer melt is

being cooled and biaxially oriented at the same time. The

ratio between the annular die diameter and the ﬁnal

bubble diameter deﬁnes the ‘‘blowup ratio’’ (BUR) for the

ﬁlm from this process. The die gap opening and BUR

deﬁne the ﬁnal ﬁlm thickness. Because the blown ﬁlm

process requires such high polymer melt strengths, higher

molecular-weight polymers must be used and are reﬂected

in the typical melt index values of o1–5 dg/min for poly-

oleﬁn resins (see Figure 3). Coextrusion-blown-ﬁlm pro-

cesses are useful for preparing multilayer ﬁlms. Care

must be taken to ensure that the tacky heat-seal layer

ends up as the outside of the bubble and not the bubble

inside, where it might stick to itself as the warm bubble is

collapsed at the nip. One advantage of the blown-ﬁlm

process is that it yields a ﬁlm tube and not a ﬂat ﬁlm with

thick edge beads. Thus blown-ﬁlm products can be more

efﬁciently used because the thicker edge beads are not

present and do not need to be removed or recycled.

Extrusion Coating. As this name implies, the extrusion

coating process is employed to apply a thin layer of polymer

ﬁlm onto an existing ﬁlm structure (see Figure 4). Thus, it

is not usually used to make structural monoextruded or

coextruded ﬁlms. It is often used to apply a thin sealant

layer to paper, foil, or plastic ﬁlm. As Figure 4 shows, this

process resembles a cast-ﬁlm process. Extrusion coating

requires that the exiting melt curtain quickly contact and

adhere to the substrate ﬁlm. Thus, the proper compatibility

and adhesion between the substrate and the extruded resin

is important as is the need for the resin to have a relatively

high melt ﬂow value (see Figure 3). Typical extrusion

coating resin melt ﬂow values can range from about 8 dg/

min up to values of several hundred decigrams per minute.

The adhesion between the melt curtain and the substrate is

aided by the pressure on the nip roll. Because the high-

melt-ﬂow polymer is hot and quickly contacts both to the

coated stock and the quench roll, the quench-roll surface is

usually machined or treated to reduce adhesion. Chill-roll

release additives can also be incorporated into the resin to

prevent sticking (see the Extrusion coating article).

BIBLIOGRAPHY

1. L. E. Nielsen, Mechanical Properties of Polymers, Reinhold,

New York, pp. 27, 31.

2. W. A. Jenkins, Packaging Foods with Plastics, Technomic

Publishing, Lancaster, PA, 1991.

3. B. A. Morris, TAPPI J. 75(8), 107–110 (1992).

General References

J. H. Briston, Plastic Films, 2nd edition, Godwin, London, 1983.

F. W. Billmeyer, Jr., Textbook of Polymer Science, Wiley, New York,

1965.

FILM, RIGID PVC

H. J. G. VAN BEEK

R. G. RYDER

Klo

ckner-Pentaplast of

America, Inc., Gordonsville,

Virginia

Poly(vinyl chloride) (PVC) is extraordinarily adaptable to

custom compounding for desired performance. This diver-

sity of physical properties at relatively low cost has been

the driving force behind its popularity in the packaging

industry.

HOMOPOLYMER PVC RESIN

Commercial PVC resin is a dry free-ﬂowing powder pro-

duced by the polymerization of vinyl chloride monomer

[see Poly(vinyl chloride].

Molecular Weight and Viscosity

The fundamental property of PVC is its molecular weight:

a measure of its means polymer chain length and a

parameter proportional to the resin’s viscosity. In general,

higher-molecular-weight (higher-viscosity) resins require

higher processing temperatures and yield ﬁlm or sheet

with higher heat-distortion temperatures, impact resis-

tance, and stiffness than do resins of lower average

molecular weight. The processing equipment for generat-

ing ﬁlm and sheet for the packaging industry demands

Figure 3. Resin meet ﬂow vs. extrusion process.

Figure 4. Extrusion coating line.

FILM, RIGID PVC 493

low- to medium-viscosity PVC resins with relative viscos-

ities of 1.75–2.10, in contrast to extrusion pipe and con-

struction markets that typically use medium- to high-

molecular-weight resins.

Heat Stability

All PVC resins are subject to thermal degradation during

processing and must be compounded with appropriate

heat stabilizers to minimize discoloration. These stabili-

zers serve to scavenge free radicals that perpetuate de-

gradation, as well as hydrogen chloride, the principal

degradation product. The highly complex thermal degra-

dation mechanism proceeds by an ‘‘unzippering’’ process

whereby liable allylic chlorides act as reaction sites for the

liberation of hydrogen chloride and the formation of

conjugate-bonding systems. When the conjugation exceeds

six bonds in length, color development begins and, if

unchecked by ample and appropriate use of heat stabili-

zers, will progress from a very subtle yellow tint to amber

and, ﬁnally, to black. Commerical processing methods

occasionally generate ‘‘burned’’ material that has reached

these initial stages of degradation. It is important to

recognize that such thermal degradation is both time-

and temperature-dependent and that although stabilizers

retard the rate of degradation during processing, they do

not prevent it. The stabilizers also help protect the ﬁlm or

sheet during subsequent processing (e.g., thermoforming)

and during the lifetime of the package itself.

COMPOUNDING FOR PROPERTIES

Among all polymers used in the packaging industry, PVC

is widely regarded as the most versatile and suitable for

custom compounding to deliver special properties (see

Additives, plastics). It may be compounded for high clarity

and sparkle or for maximum opacity and it accepts a full

range of custom colorants (see Colorants). Properly com-

pounded, PVC ﬁlm and sheet are approved for food and

drug contact and are available with residual vinyl chloride

monomer (VCM) levels below 10 ppb. Examples of what

custom compounding can produce are shown in Table 1.

After identiﬁcation of such desired properties for the

speciﬁc packaging application, the compounder selects a

suitable resin viscosity. If the ﬁlm is to be approved

for food or drug contact, the resin’s VCM level must be

low enough before processing to ensure that the resulting

ﬁlm will meet all customer requirements on residual

VCM. A heat stabilizer must then be selected. Tin mer-

captides are frequently chosen because of their high

efﬁciency, excellent early color, good light stability, and

excellent crystal clarity in the product (1). Some of these

stabilizers (octyl tins) are cleared for food and drug

contact. Uncleared options include lead stabilizers, which

are limited to opaque systems, and combinations of

barium, cadmium, and zinc. Although there are a few

calcium/zinc systems in limited use in food packaging,

the tin stabilizers dominate the packaging ﬁeld. Octyl tins

are the principal systems used in food and pharmaceu-

tical packaging (2). Stabilizers are also available that

impart improved UV resistance. All of the stabilizers

mentioned above are used in rigid PVC at only very small

loadings.

In contrast, impact modiﬁers may constitute up to 15%

of the product’s weight. As a result, the proper selection

and loading of impact modiﬁer is an important compound-

ing decision. Clear packaging ﬁlms typically contain MBS

impact modiﬁers because of their superior clarity, heat

stability, and room-temperature efﬁciency. ABS modiﬁers

are good for opaque products, and chlorinated polyethy-

lene (CPE) and acrylics (see Acrylics) are often selected for

outdoor applications and/or low-temperature environ-

ments in opaque systems. Pigments may then be added

to provide custom color, and titanium dioxide is generally

used at levels as high as 15% to provide the desired level of

opacity. Fillers may be used for cost reduction in opaque

systems and, in many cases, to improve such physical

properties as impact strength, stiffness, and heat-distor-

tion temperature. Present in very low levels are a variety

of proprietary lubricants and processing aids that are

necessary to facilitate processing and to provide desirable

properties such as slip, denesting, and improved thermo-

formability for the ﬁlm processor/packager. Flame retar-

dants, antioxidants, coupling agents, antistatic agents,

phosphite stabilizers, and a host of additional additives

may be included if necessary. Because of this tremendous

facility for custom compounding, and the variety of pro-

ducts that result from it, Table 2 must be considered only

as a general guide to typical rigid PVC properties.

FILM AND SHEET PRODUCTION METHODS

Extrusion (see Extrusion) and calendering are the princi-

pal methods of producing rigid PVC for the packaging

industry. Extrusion is used to produce very thin blown

ﬁlms (see Film, ﬂexible PVC) as well as heavy-gauge

sheeting nearly 1 in. (2.54 cm) thick produced by sheet-

die methods. Calendering requires a much greater capital

investment, but it offers much greater production rates,

superior gauge control (cross direction and machine direc-

tion) (75%), superior cosmetic quality including clarity,

and much wider versatility in accommodating gauge and

Table 1. Examples of Custom Compounding

Higher heat-distortion temperatures for hot-ﬁll packaging

Ethylene oxide (ETO) sterilizable ﬁlm without water-blush for

medical devices

Improved low-temperature impact resistance for drop tests of

shipping cartons

Improved UV resistance

Improved outdoor weatherability

Improved sealability (impulse heat, RF, ultrasound)

Denesting formulations for machine-fed blisters

Static-resistant formulations

Optimum performance in laminating to other materials (PVDC,

PE, etc.)

Formulations for vacuum metalizing

Improved printability

Absence of ‘‘white break’’ or crease whitening

494 FILM, RIGID PVC

width changes. Calendered ﬁlm and sheet generally have

better dimensional stability, which provides thermoform-

ing consistency throughout a given lot. Rigid calendered

PVC is available in thickness of 2–45 mil (51–1143 mm)

with gloss, matte, or embossed surfaces, either in rolls or

in sheets up to about 60 in. (1.5 m) wide. Calendering is

the principal means of processing rigid PVC ﬁlm for

packaging.

Calendering

In a modern rigid PVC calendering operation, compound-

ing is done by computer-controlled electronic scales

that supply precise amounts of each ingredient to a

high-intensity mixer designed to incorporate all liquids

into the resin particles and to secure uniform distribution

of all powdered ingredients. Blending is generally

done for a speciﬁc time period and to a speciﬁc tempera-

ture. The still-dry, free-ﬂowing blend is then charged to a

feed hopper where it is screw-fed into a continuous mixer

such as an extruder or kneader. Under the action of this

mixer’s reciprocating screw in the conﬁned volume of the

mixing chamber, the blend begins to ﬂux or masticate

into the plastic state. It is then forced out of the barrel

of the mixing chamber. The continuous strand may be

chopped into small ﬁst-sized buns of hot material or

simply exit as a continuous rope. This material may

then be directly conveyed to the calender, or it may

ﬁrst pass through a two-roll mill. The calender is a large

unit, typically consisting of four or ﬁve heated rolls

designed to process masticated PVC buns into a con-

tinuous web of designated width and thickness (see

Figure 1).

Table 2. Typical Physical Properties of Rigid PVC (Clear)

Property Test Method

Units Values

Speciﬁc gravity

D1505 1.30–1.36

Yield (1.30 sp gr) D1505 in.

lb (cm

/g)

7.5 mil (0.19 mm) 2850 (40.5)

10.0 mil (0.25 mm) 2130 (30.3)

12.0 mil (0.30 mm) 1780 (25.3)

15.0 mil (0.38 mm) 1420 (20.2)

20.0 mil (0.51 mm) 1070 (15.2)

Tensile strength (yield) D882 psi (MPa) 6500–7800 (44.8–53.8)

Tensile modulus D882 psi (MPa) 2.5–4.0 10

(1723–2757)

Elongation (break) D882 % 180–220

Izod impact (1/4 in. or 6.4 mm) D256 ft lbf in (J/m) 0.5–20.0 (26.7–1068)

Gloss, 201 D247 120–160

Heat-distortion temperature (264 psi or 1.82 MPa) D648

F(1C) 158–169 (70–76)

Cold-break temperature D1790

F(1C) 14 to 40 (10 to 40)

WVTR (381C, 90% rh) DIN53122 g (100 in.

24 h)

[g (m

day)]

7.5 mil (0.19 mm) 0.30 [4.7]

10.0 mil (0.25 mm) 0.20 [3.1]

Surface resistance DIN53482 O 10

–10

Speciﬁc resistance DIN40634 O cm 10

–10

Dielectric strength DIN40634 kV/mm 60–70

Speciﬁc heat (201C) kJ/(kg K) 0.8

Thermal conductivity W/(m K) 0.16

Linear thermal expansion K

1

7.0–8.0 10

5

Infrared absorption

(3–18 mm) Various intensities

Ds are ASTM test methods, and DINs are German (Deutsche) Industr ial Norm test methods.

Indirectly related to amount of impact modiﬁer. Increased opacity may raise to 1.40.

20 mil (508 mm) unmodiﬁed.

Figure 1. PVC calender.

FILM, RIGID PVC 495

Figure 2 illustrates the typical ‘‘L’’ and ‘‘inverted L’’

conﬁgurations generally used for rigid and ﬂexible pro-

duction, respectively (3). The calender rolls have separate

temperature and speed controls as well as roll bending

and crossing capabilities to control proﬁle across the web.

Proper use of these controls, along with speed and stretch

in the takeoff train, allows the production of an extremely

ﬂat sheet with a proﬁle tolerance of less than 75% across

and down the web. Such control is maintained by contin-

uous beta scanning equipment that traverses the web

constantly and calls for adjustments in nip openings,

skew, and/or roll bending. Such constant inprocess mon-

itoring and continuous proﬁle adjustments is a signiﬁcant

advantage of calendering over other processing methods.

By using special grit-blasting techniques, the third and

fourth calender rolls may be custom-surfaced to generate

a uniform two-sided matte product. Alternatively, one or

more downstream embossing stations may be utilized to

produce a custom surface on one or both sides of the ﬁlm.

Antistatic and or de-nesting slip agents may be applied to

the surface(s) of the web after separating from the last

calender roll. Finally, after the cooling section, the web is

cut in-line into ﬁnished sheets or wound about a core into

a master roll for subsequent custom slitting. Typical slit

widths are made to the nearest 1/32 in. (0.8 mm) on 3- or

6-in. (7.6- or 15.2-cm) cores with roll diameters of 14–40 in.

(36–102 cm).

PACKAGE PRODUCTION BY THERMOFORMING

Most commercial PVC packages are the result of thermo-

forming rollstock into custom blisters. In those cases

where further enhancement of PVC’s own oxygen and/or

moisture barrier properties are required, barrier materi-

als (see Barrier polymers) such as PE, PVDC, or ﬂuor-

opolymer ﬁlm (see Film, ﬂuoropolymer) may ﬁrst be

laminated to the PVC web prior to thermoforming. Ther-

moforming processing conditions are generally dictated by

the PVC material itself regardless of lamination. Since

PVC is an amorphous material (see Polymer properties), it

softens over a large temperature range and has no sharp

melting point. There are two temperatures ranges in

which rigid PVC can most readily be formed (see Figure

3). It must be emphasized that these temperatures are

actual ﬁlm temperatures that must be measured with

thermocouples located directly on the surface of the ﬁlm

(heating-element temperatures are very much hotter).

The ﬁrst plateau at 221–2751F (105–1351C) is the

elastic area best for most high-speed form/ﬁll/seal pres-

sure-forming machines (see Thermoform/ﬁll/seal). In this

area, the ﬁlm has sufﬁcient hot strength to elongate or

stretch to the conformation of the mold. Between 2751F

and 3381 F (1351C and 1701 C) is a region of inadequate

elongation for proper forming; and attempts to process in

this area may result in blowing holes, tearoffs, and poor

Figure 2. PVC calender operation.

496 FILM, RIGID PVC

deﬁnition. Failure to respect this ‘‘no man’s land’’ is among

the most common reasons for thermoforming problems

seen in the ﬁeld by PVC technical service representatives.

The elongation quickly increases in the ﬁlm at 3381F

(1701C), and excellent forming is possible up to about

3651F (1851C). This is the range for optimum performance

for conventional commercial thermoforming machines

(see Thermoforming). Beyond 3651F (1851C), the material

will sag excessively, resulting in webs, wrinkles, holes, or

thin areas. In general, rigid PVC ﬁlm thermoforms best

when the ﬁlm temperature is slightly above 3381F

(1701C), with relatively cool molds (as low as 50–601For

10–161C), and when plug assist temperatures are about

1941C (901C). Mold design should avoid exceeding a 1:1

draw ratio, and a few extra degrees of draft and extra

radius on corners will help avoid problems.

Film Shrinkage for Optimum Forming

Since distinct types of equipment are used in each of

the two forming regions, it is particularly important that

the PVC-ﬁlm supplier have a thorough understanding

of the appropriate shrinkages to be put into the feedstock

for each type of machine. PVC destined for pressure-

forming over the lower temperature region should

have its shrinkage controlled by the producer at 2841F

(1401C), a temperature that will release all plastic mem-

ory relevant to the processing range of 221–2751F

(105–1351C). Typically, best results are achieved with

PVC ﬁlm having 2841F (1401C) shrinkages of 2–5% in

the machine direction and 0–1% in the transverse direc-

tion. Such very slight growth in the transverse direction

is desirable to compensate for the repeating necking-in

that may otherwise occur across each line of blisters such

as those used for press-through packaging of ethical

drugs.

In the higher temperature range for conventional

thermoforming, ﬁlm shrinkages should be controlled by

the ﬁlm supplier at 3501F (1771C), a temperature that will

release all plastic memory relevant to this higher proces-

sing range. Best results are typically achieved with PVC

ﬁlm having 3501F (1771C) shrinkages of 4–8% in the

machine direction and 1 to +1% in the transverse direc-

tion. Excessive shrinkages may cause the ﬁlm to pull out

of the chains, clips, or frame. Insufﬁcent machine direction

shrinkage and/or excessive transverse growth may led to

webbing. Thermoforming machine operators often re-

spond to such webbing problems by reducing operating

temperatures to prevent excessive sag, but this step can

result in blowing holes and poor wall distribution because

it takes the ﬁlm out of the optimum thermoforming

temperature range and elongation falls off rapidly. Careful

shrinkage control is critical to successful thermoforming

of rigid PVC ﬁlm with all types of forming equipment.

Proprietary process controls are available in modern PVC

calendering technology to custom produce ﬁlm with the

proper shrinkages for optimum thermoformability within

each temperature region and on each design of forming

equipment.

PACKAGING MARKET

U.S. consumption of calendered rigid PVC ﬁlm and sheet

in 1994 is estimated at 523 

lb (237,000 tons), not

including an additional 30% for extruded sheet. About

63% of this calendered production was used in packaging

(see Table 3). Total 1994 consumption of calendered rigid

PVC ﬁlm and sheet for packaging was 330 10

(150,000 tons).

Figure 3. Temperature ranges for PVC thermoforming.

Table 3. U.S. Markets for Calendered Rigid PVC Film

and Sheet

Market 10

(10

tons)

Packaging (FDA-grade)

Food 25 (11.4)

Pharmaceuticals 30 (13.6)

Medical devices and supplies 10 (4.5)

Total packaging (FDA grade) 65 (29.5)

Packaging (general-purpose)

Thermoforming—blisters, clam

shells, etc.

225 (102.3)

Vinyl boxes and lids 35 (15.9)

Static control thermoforms 5 (2.3)

Total general-purpose and specialty

packaging

265 (120.5)

Specialty and industrial applications

Printing and stationery 90 (40.9)

Cooling tower and wastewater ﬁll

media

40 (18.2)

Artiﬁcial Christmas trees 30 (13.6)

Furniture lamination 10 (4.5)

Construction and ﬂooring 10 (4.5)

Floppy disks 8 (3.6)

Litho mask 5 (2.3)

Total specialty and industrial

applications

193 (87.6)

Total packaging (61.5%) 330 (150.0)

Total nonpackaging (36.5%)

193 (87.7)

Total rigid PVC ﬁlm and sheet 523 (237.7)

Source: Klo

ckner-Pentaplast of America, Inc.

FILM, RIGID PVC 497